Writing Dockerfiles

Create production-grade Dockerfiles with multi-stage builds, security hardening, and language-specific optimizations.

When to Use

Invoke this skill when:

• "Write a Dockerfile for [Python/Node.js/Go/Rust] application"
• "Optimize this Dockerfile to reduce image size"
• "Use multi-stage build for..."
• "Secure Dockerfile with non-root user"
• "Use distroless base image"
• "Add BuildKit cache mounts"
• "Prevent secrets from leaking in Docker layers"

Quick Decision Framework

Ask three questions to determine the approach:

1. What language?

• Python → Use poetry/pip with virtual environments
• Node.js → Use npm ci with alpine images
• Go → Static binary + distroless = 10-30MB
• Rust → musl static linking + scratch = 5-15MB

2. Is security critical?

• YES → Use distroless runtime images
• NO → Use slim/alpine base images

3. Is image size critical?

• YES (<50MB) → Multi-stage + distroless + static linking
• NO (<500MB) → Multi-stage + slim base images

Core Concepts

Multi-Stage Builds

Separate build environment from runtime environment to minimize final image size.

Pattern:

# Stage 1: Build
FROM build-image AS builder
RUN compile application

# Stage 2: Runtime
FROM minimal-runtime-image
COPY --from=builder /app/binary /app/
CMD ["/app/binary"]

Benefits:

• 80-95% smaller images (excludes build tools)
• Improved security (no compilers in production)
• Faster deployments
• Better layer caching

Base Image Selection

Language	Build Stage	Runtime Stage	Final Size
Go (static)	golang:1.22-alpine	gcr.io/distroless/static-debian12	10-30MB
Rust (static)	rust:1.75-alpine	scratch	5-15MB
Python	python:3.12-slim	python:3.12-slim	200-400MB
Node.js	node:20-alpine	node:20-alpine	150-300MB
Java	maven:3.9-eclipse-temurin-21	eclipse-temurin:21-jre-alpine	200-350MB

Distroless images (Google-maintained):

• gcr.io/distroless/static-debian12 → Static binaries (2MB)
• gcr.io/distroless/base-debian12 → Dynamic binaries with libc (20MB)
• gcr.io/distroless/python3-debian12 → Python runtime (60MB)
• gcr.io/distroless/nodejs20-debian12 → Node.js runtime (150MB)

BuildKit Features

Enable BuildKit for advanced caching and security:

export DOCKER_BUILDKIT=1
docker build .
# OR
docker buildx build .

Key features:

• --mount=type=cache → Persistent package manager caches
• --mount=type=secret → Inject secrets without storing in layers
• --mount=type=ssh → SSH agent forwarding for private repos
• Parallel stage execution
• Improved layer caching

Layer Optimization

Order Dockerfile instructions from least to most frequently changing:

# 1. Base image (rarely changes)
FROM python:3.12-slim

# 2. System packages (rarely changes)
RUN apt-get update && apt-get install -y build-essential

# 3. Dependencies manifest (changes occasionally)
COPY requirements.txt .
RUN pip install -r requirements.txt

# 4. Application code (changes frequently)
COPY . .

# 5. Runtime configuration (rarely changes)
CMD ["python", "app.py"]

BuildKit cache mounts:

RUN --mount=type=cache,target=/root/.cache/pip \
    pip install -r requirements.txt

Cache persists across builds, eliminating redundant downloads.

Security Hardening

Essential Security Practices

1. Non-root users

# Debian/Ubuntu
RUN useradd -m -u 1000 appuser && chown -R appuser:appuser /app
USER appuser

# Alpine
RUN adduser -D -u 1000 appuser && chown -R appuser:appuser /app
USER appuser

# Distroless (built-in)
USER nonroot:nonroot

2. Secret management

# ❌ NEVER: Secret in layer history
RUN git clone https://${GITHUB_TOKEN}@github.com/private/repo.git

# ✅ ALWAYS: BuildKit secret mount
RUN --mount=type=secret,id=github_token \
    TOKEN=$(cat /run/secrets/github_token) && \
    git clone https://${TOKEN}@github.com/private/repo.git

Build with:

docker buildx build --secret id=github_token,src=./token.txt .

3. Vulnerability scanning

# Trivy (recommended)
trivy image myimage:latest

# Docker Scout
docker scout cves myimage:latest

4. Health checks

HEALTHCHECK --interval=30s --timeout=3s --start-period=10s --retries=3 \
  CMD wget --no-verbose --tries=1 --spider http://localhost:8080/health || exit 1

.dockerignore Configuration

Create .dockerignore to exclude unnecessary files:

# Version control
.git
.gitignore

# CI/CD
.github
.gitlab-ci.yml

# IDE
.vscode
.idea

# Testing
tests/
coverage/
**/*_test.go
**/*.test.js

# Build artifacts
node_modules/
dist/
build/
target/
__pycache__/

# Environment
.env
.env.local
*.log

Reduces build context size and prevents leaking secrets.

Language-Specific Patterns

Python: Poetry Multi-Stage

FROM python:3.12-slim AS builder
RUN --mount=type=cache,target=/root/.cache/pip \
    pip install poetry==1.7.1

COPY pyproject.toml poetry.lock ./
RUN poetry export -f requirements.txt --output requirements.txt

RUN --mount=type=cache,target=/root/.cache/pip \
    python -m venv /opt/venv && \
    /opt/venv/bin/pip install -r requirements.txt

FROM python:3.12-slim
COPY --from=builder /opt/venv /opt/venv
ENV PATH="/opt/venv/bin:$PATH"
USER 1000:1000
CMD ["python", "-m", "uvicorn", "main:app", "--host", "0.0.0.0"]

Node.js: Express Multi-Stage

FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN --mount=type=cache,target=/root/.npm \
    npm ci
COPY . .
RUN npm run build
RUN npm prune --omit=dev

FROM node:20-alpine
WORKDIR /app
COPY --from=builder /app/node_modules ./node_modules
COPY --from=builder /app/dist ./dist
USER node
CMD ["node", "dist/index.js"]

Go: Distroless Static

FROM golang:1.22-alpine AS builder
WORKDIR /app
COPY go.mod go.sum ./
RUN --mount=type=cache,target=/go/pkg/mod \
    go mod download

COPY . .
RUN --mount=type=cache,target=/go/pkg/mod \
    --mount=type=cache,target=/root/.cache/go-build \
    CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o main .

FROM gcr.io/distroless/static-debian12
COPY --from=builder /app/main /app/main
USER nonroot:nonroot
ENTRYPOINT ["/app/main"]

Final image: 10-30MB

Rust: Scratch Base

FROM rust:1.75-alpine AS builder
RUN apk add --no-cache musl-dev
WORKDIR /app

# Cache dependencies
COPY Cargo.toml Cargo.lock ./
RUN --mount=type=cache,target=/usr/local/cargo/registry \
    mkdir src && echo "fn main() {}" > src/main.rs && \
    cargo build --release --target x86_64-unknown-linux-musl && \
    rm -rf src

# Build application
COPY src ./src
RUN --mount=type=cache,target=/usr/local/cargo/registry \
    cargo build --release --target x86_64-unknown-linux-musl

FROM scratch
COPY --from=builder /app/target/x86_64-unknown-linux-musl/release/app /app
USER 1000:1000
ENTRYPOINT ["/app"]

Final image: 5-15MB

Package Manager Cache Mounts

Language	Package Manager	Cache Mount Target
Python	pip	--mount=type=cache,target=/root/.cache/pip
Python	poetry	--mount=type=cache,target=/root/.cache/pypoetry
Python	uv	--mount=type=cache,target=/root/.cache/uv
Node.js	npm	--mount=type=cache,target=/root/.npm
Node.js	pnpm	--mount=type=cache,target=/root/.local/share/pnpm/store
Go	go mod	--mount=type=cache,target=/go/pkg/mod
Rust	cargo	--mount=type=cache,target=/usr/local/cargo/registry

Persistent caches eliminate redundant package downloads across builds.

Common Patterns Quick Reference

1. Static binary (Go/Rust) → Smallest image

• Build: Language-specific builder image
• Runtime: gcr.io/distroless/static-debian12 or scratch
• Size: 5-30MB

2. Interpreted language (Python/Node.js) → Production-optimized

• Build: Install dependencies, build artifacts
• Runtime: Same base, production dependencies only
• Size: 150-400MB

3. Security-critical → Maximum hardening

• Base: Distroless images
• User: Non-root (nonroot:nonroot)
• Secrets: BuildKit secret mounts
• Scan: Trivy/Docker Scout in CI

Anti-Patterns to Avoid

❌ Never:

• Use latest tags (unpredictable builds)
• Run as root in production
• Store secrets in ENV vars or layers
• Install unnecessary packages
• Combine unrelated RUN commands (breaks caching)
• Skip .dockerignore (bloated build context)

✅ Always:

• Pin exact image versions (python:3.12.1-slim, not python:3)
• Create and use non-root user
• Use BuildKit secret mounts for credentials
• Minimize layers and image size
• Order commands from least to most frequently changing
• Create .dockerignore file

Related Skills

• Building CI Pipelines - Build and push Docker images in CI
• Implementing GitOps - Deploy containers to K8s clusters
• Testing Strategies - Test application before containerizing

References

• Full Skill Documentation
• Base Image Selection
• BuildKit Features
• Security Hardening
• Language Examples